1. Field of the Invention
This invention relates to an optical module.
2. Description of the Related Art
JP 2013-004787 A discloses an optical module including a stem formed using a conducting material and connected to the ground, a lead terminal penetrating the stem and attached to the stem and electrically insulated from the stem via an insulating material in a part penetrating the stem, and a photonic device mounted on a stem mount as a part of the stem and electrically connected to the lead terminal, wherein a dielectric substrate having a front surface opposed to the lead terminal and a rear surface electrically connected to the stem mount.
JP 2005-191088 A discloses a package for semiconductor having a high-frequency transmission line penetrating a package casing for transmitting a high-frequency signal from outside to inside, the high-frequency transmission line including a first transmission line part provided in a partition wall part from outside of the package and having a lower characteristic impedance than a characteristic impedance to be matched when transmitting the high-frequency signal from outside to inside, a second transmission line part provided outside of the package, electrically connected to the first transmission line part, and having a higher characteristic impedance than the first transmission line part, and a third transmission line part provided inside of the package, electrically connected to the first transmission line part, and having a higher characteristic impedance than the first transmission line part.
JP 2012-227486 A discloses an optical device including a mounting part having a stem, an upper surface, a lower surface, a first surface and a second surface corresponding to side surfaces with respect to the upper surface and the lower surface, the lower surface connected to the stem, a photonic device mounted on the upper surface of the mounting part, electronic components respectively mounted on the first surface and the second surface of the mounting part, and a plurality of first leads penetrating the stem and led out to the first surface side and the second surface side of the mounting part, and electrically connected to the electronic components.
Today, most of the Internet and telephone networks are constructed by optical communication networks. Optical modules used as routers/switches as optical communication devices and interfaces of transmission apparatuses have important roles of converting electrical signals into optical signals.
Downsizing is required for the optical module itself and, for realization of further speeding up, mounting having higher reliability is required for the optical module.
For example, as an optical module that fulfills the above described requirements, e.g. a CAN package optical module or the like having a form in which lead terminals to be inserted into a printed board or the like from a metal stem contained in a box-shaped can package protrude is known.
The optical module includes a photonic device for converting one of an optical signal and an electrical signal into the other. Here, for the electrical signal, e.g. a differential signal including a normal phase signal and a reverse phase signal may be used. In this case, there is a problem in realization of impedance matching in a transmission line.
Placement of several relay boards in a transmission line for the electrical signal to reach the photonic device is not avoidable, inductance components are parasitic in connecting parts to the relay boards, and consequently, impedance mismatch is caused.
Therefore, for realization of impedance matching, it is necessary to compensate for the inductance components generated in the connecting parts by capacitances. For example, compensation by incorporation of a chip capacitor in the transmission line of the relay board is considered, however, adverse effects of upsizing of the relay boards and cost increase for placement of chip capacitor are caused.